As for semiconductor elements mounted on a wiring board, the number of connection terminals thereof is increasing, and also the pitch between each of the connection terminals is more and more narrowing. Flip-chip mounting is a mounting technique to cope with such an increased number of connection terminals and narrowed pitch.
In the flip-chip mounting, connection terminals made of solder bumps or the like are made to reflow, and thereby a wiring board and electrodes of a semiconductor element are connected to each other through the connection terminals.
The reflow of the connection terminals involves heating of the semiconductor element and the wiring board. However, a difference in thermal expansion coefficient between the semiconductor element and the wiring board may cause the semiconductor element to be damaged during the reflow.
In order to inhibit such damage, it is effective to use a material having as low a melting point possible as a material for the connection terminals made of solder bumps or the like, and thereby lower the heating temperature during the reflow.
Lead-free solder is widely used as a material for the connection terminals, but is not suitable for such low-temperature reflow because of its high melting point. A Sn—Ag—Cu-based lead-free solder, for example, has a high melting point of 217° C.
Therefore, a Sn—Bi-based solder having a low melting point of 139° C. and having a eutectic point composition is often used as the material for the connection terminals.
However, the connection terminals formed using the Sn—Bi-based solder still have room for improvement in terms of the enhancement of the bonding strength with the electrodes.
Note that techniques related to the present application are disclosed in Japanese Laid-open Patent Publications Nos. 2012-157873 and 2010-167472.